Filters for heating, ventilation, and air conditioning (HVAC) systems play an important role by minimizing deposits of dust and other particles on cooling coils and heating surfaces. Deposits of dust or other materials on cooling coils and heating surfaces reduce the efficiency, and therefore increase the energy needed to operate the HVAC system. A clogged filter can result in decreased air flow over cooling coils and heating surfaces, and decreased air flow also lowers efficiency and can shorten the life of a HVAC system because the HVAC system has to run longer to maintain the desired temperature.
After a period of use, dust accumulates on the filters, causing them to become clogged and requiring additional energy consumption. At some point, it becomes cost-effective to remove the filter and either clean it or replace it with a new one. HVAC equipment manufacturers typically state their warranties so that the user, not the manufacturer, is responsible for equipment failures due to neglect in maintaining the system. Manufacturers typically emphasis maintenance of the dust filters. In most cases, the manufacture does not have access to an operating HVAC system unless called by the user, so it is not practical for a manufacturer to be responsible for filter maintenance. It often takes weeks or months before an air filter needs to be cleaned or replaced, and the long periods of time and lack of constant attention can result in users neglecting air filter maintenance.
Many newer, high efficiency air-conditioners and heat pumps now use variable speed fan motors (ECM—Electronically Commutated Motors) which attempt to achieve constant, optimized flow through the dust filter and AC or heat pump coils, even as dust gradually clogs the filter. The filter can, never-the-less, eventually clog sufficiently that the air flow decreases. The increased fan motor power required to draw air through the clogging filter decreases the HVAC efficiency. Decreasing flow across the coils below the optimum flow rate also decreases the system efficiency. Increases in run time due to the clogged filter can both 1) increase energy requirements (decrease efficiency) and 2) decrease system lifetime, because HVAC system lifetime approximates inverse proportionality to system run-time.
In recent years, the lower costs and increased accuracies of differential pressure switches have led to their more frequent application for monitoring the clogging of dust filters. These instruments monitor differential pressure between the input and output sides of the filter. When the differential pressure reaches a predetermined level, an alarm is triggered which indicates the filter is clogged. Problems can occur from opening/closing of doors between rooms or other actions modifying ambient room pressure. Any ducting leaks, quite normal in older systems, also leads to increased flow of non-filtered (filter bypass) air as the filter becomes clogged. With increase dust loading and clogging of the dust filter, the filter will often physically warp so that air flows around the filter; i.e., a filter bypass air-flow path is created until the filter is cleaned or replaced.
An electro-optical dust filter sensor/alarm offers advantages over the differential pressure sensor in that it is not affected by the flow across the filter or by the speed of the HVAC fan motor. Since the invention of dust filters, the “eyeball” (visually examining the dust/dirt build-up on the filter's leading edge) has been the traditional means of determining when the filter needs replacing or cleaning. This eyeball examination is often left to the memory of the user, which may be aided by such things as a calendar date (i.e., the first of each month).
Some electro-optical based clogged dust filter detectors are designed for use with low optical density filter media. However, a filter detector which functions with filters designed for smaller (sub-micron) particle removal is desirable. These filters are necessarily more optically dense than traditional low optical density filter media, and therefore require an optical filter detector that has increased optical sensitivity.
One example of an optical filter detector comprises a simple light emitting diode (LED) coupled to a receiving photo-diode (or photo-transistor) with a simple signal processor. This filter detector may perform the required task with the lower cost, lower optical density, lower MERV (minimum efficiency reporting value) rated dust filters. However, as the filters become more optically dense to remove smaller (few micron to submicron) particles (i.e., filters with higher MERV values), the simple optical transmitter/sensor electro-optical filter detectors are not sufficient.
The cheaper, lower MERV filters typically either have no pleats or pleats with a spacing of 1 to 2 pleats (folds) per inch; at least some of the high MERV filters are manufactured with a pleat spacing of up to 8 pleats per inch. Thus the high MERV filters are more optically dense, especially when viewing across the pleats. Therefore the basic electro-optical system described above may not be suitable with the high MERV, optically dense filters such as the 3M® Filtrete® 1500, 1700 or 1900 brand or equivalent DuPont® filters. As stated above, high MERV filters remove a greater percentage of small (micron and submicron) particles than the cheaper filters. This is especially important for people with allergies or in clean room or sterile hospital situations requiring “clean air”.
In order to operate with higher MERV, high efficiency (from small particle collection standpoint) filters, the filter detector must thus have adequate sensitivity to function with optically dense filter media. Also, since HVAC dust filters are often located in hallways with no available electrical power, a long-life, battery-powered filter detector can be desirable.
Filters can be used for a wide variety of purposes other than the HVAC systems discussed above. For example, filters are used to control incoming dust and outgoing paint particles for paint spraying operations. They are also used to control dust accumulation on electronic components for ventilated electronics enclosures such as those used by cable television companies on the side of the street. The clean environment used in fabrication of micro-electronic chips typically requires a filtered air source. Very clean (filtered) air is also required for some medical applications. These filter uses can all be referred to as HVAC (heating, ventilation and air-conditioning) applications.
Since the filter maintenance requirement is environment driven rather than time driven, a clogged filter sensor is helpful in maintaining HVAC system optimal operating efficiency. A system that notified the user when an air filter needed attention could minimize energy losses and decreased performance resulting from clogged air filters.